Expanded cellular or foamed thermoplastic cups and containers have been known in the art for some time. They are normally moulded from expanded polystyrene, prepared from expanded polystyrene beads, initially containing approximately 5% by weight of hydrocarbon blowing agent. The closed cell structure of the expanded polystyrene cups, for example, is made at a density of around 4 to 6 pounds per cubic foot, with an internal volume of about 200 cubic centimeters and a weight from slightly less than about two grams to greater than about three grams to provide a desired degree of thermal insulation. The processing of the cups from the polystyrene beads involves pre-expansion of the beads, the addition of a small percentage by weight of a moulding/fusion lubricant (commonly zinc stearate), the ageing of the beads under regulated conditions (for example, if the expansion takes place in a hot air pre-expander, it may be dried and aged in dry conditions for between about 6 to about 24 hours) to allow equilibration of the blowing agent, and finally, the deposition of the pre-expanded beads into a cup mould for fusion by the application of heat for example, by the use of dry steam. During the moulding operation, the initially spherical pre-expanded polystyrene beads expand further and fuse to each other, assuming for example, a dodecahedral shape, yielding a leakproof moulded cup.
Conventional zinc stearate lubricated, moulded, expanded polystyrene cups exhibit dielectric characteristics similar to polystyrene and are readily statically charged, causing deposited powdered comestibles to stick to the sides and nested cups in a stack to stick together preventing their separation when one is to be dispensed from the stack.
In many instances, the electrostatic force between the cups is greater than the stripping or discharge force applied to drop the lowermost cup from the stack of cups. Particularly, the expanded, cellular or foamed thermoplastic cups which weigh in the order of about 1.8 grams are impeded by the slightest electrostatic forces keeping the cups together, from dropping from the stack of cups down the chute. Additionally, because of the static charge adhering to the cup side wall, comestible deposited in the lower cup tends to cling to the inner side wall so that comestible in contact with the outer side wall of the upper adjacent cup is transferred to the outer side wall of the cup.
Expanded polystyrene beads hold static charges for a long time, sometimes months. Where the expanded beads are in containers or cups to hold powdered comestibles for mixture with water, it is desirable that the anti-static charges be dissipated. In this regard, the surface conductivity must be changed. Where molecules on the surface for providing anti-static characteristics absorb water, the static charge is dissipated. Therefore, the molecules must have hydrophillic bonds in order to dissipate anti-static charges. However, where the anti-static material or agent is too hydrophillic, it allows the water to wet the beads too much and therefore, the water goes through. On the other hand, where the anti-static agent is too hydrophobic, i.e. does not have sufficient water-attracting molecules, then there is little done by the said agent to dissipate the static charge in the material. Therefore, a balance between hydrophillic bonds and hydrophobic bonds must be made in the anti-static agent.
A number of approaches have been proposed to reduce or eliminate static entirely. U.S. Pat. No. 3,206,429 teaches improved polyethylene compositions containing N,N-diethanol oleamide which compositions are purported to exhibit improved anti-static properties. The compositons can be prepared by dissolving the N,N-diethanol oleamide in isopropanol or other suitable solvent and externally coating polyethylene pellets by thoroughly mixing the pellets and solution in any suitable tumbling or stirring-type mixer. The coated pellets are then formed by any of the conventional methods into moulded objects or film exhibiting the desired anti-static properties.
This approach was carried over in U.S. Pat. No. 3,236,681 to the manufacture of articles consisting of foamed thermoplastic materials (in one example, expandable polystyrene particles). U.S. Pat. No. 3,236,681 discloses a process of coating expandable thermoplastic polymer particles with a film of an aqueous solution of an anti-static agent comprising a salt of an alkyl diacid phosphate of the formula X(RHPO.sub.4)n, where X may be for example, ammonium or alkali metal, n may be 1-3, and R may be an alkyl moiety containing 1-18 carbon atoms (for example, ammonium amyl hydrogen phosphate). The coated particles are then dried and then prefoamed by passing them under an infrared heater.
As a critical feature of the invention, the patent teaches deposition of at least 0.01% of the salt of an alkyl diacid phosphate on the surfaces of the expandable polymer particles based on the weight of the polymer particles with the preferred quantity being in the range of about 0.01% to about 0.1%. Where it is desired to have deposited thereon in the dried state an amount in excess of 0.10% of the salt of an alkyl diacid phosphate, the Patent teaches the incorporation of a sulfate salt with the anti-electrostatic agent, to prevent agglomeration.
While U.S. Pat. No. 3,236,681 teaches the coating of expandable polystyrene particles with an anti-static agent and prefoaming the coated particles for further processing, U.S. Pat. No. 3,595,464 specifically relates to foamed polystyrene cups and teaches the coating of the particles with a detergent (for example, aluminum lauryl sulfate and an organic amide builder) by for example, tumbling the beads in some sort of container and adding the detergent in liquid form, for fully coating the entire surface of all the beads, prefoaming the coated beads and then charging them into a mould for manufacturing a foam polystyrene cup. The Patent then teaches that by this process, the surfaces of foamed polystyrene cups may be considered to be covered with film as well as the interface between adjacent fused beads through the walls and bottoms of the cups. However, where solutions are tumbled with the bead to coat them and the coated beads are dried, some if not a substantial part of the coating agent will evaporate and consequently, there will be very little of the coating agent left to uniformly coat the beads when pre-expanded. Therefore, it is apparent that where the detergent is shown as a coat or film on the beads, it will not remain where it is when dried. Some will evaporate and some will be driven off.
Furthermore, the detergent or any detergent would be too hydrophillic, its purpose being to break down any fats and thus it would attract substantial water molecules and cause any cup to leak like a sieve, passing water therethrough.
U.S. Pat. No. 3,796,366 discloses a different approach, teaching the application of an aqueous non-ionic emulsion of polyethylene and thence subjecting the article to drying conditions to remove substantially all the water therefrom.
U.S. Pat. No. 3,796,366 relates to anti-static plastic articles which incorporate a non-ionic compound of the formula: ##STR1## wherein R is an alkyl group having 8 to 9 carbon atoms and n is an average number from 7 to 15.
U.S. Pat. No. 4,438,058 corresponds to United Kingdom Application No. 20 977 97 and DE Application No. 3,208,485 and provides a process for blowing expandable resin particles which comprises expanding resin particles in the presence of surfactants in the amount of 0.5 to 10% by weight based on the particles. However, the inventor only discloses in his examples, beads that have first been coated, then pre-expanded with steam. Once again, if the beads are coated, then dried, anti-static agent is driven off.
The German article by Dr. S. Riethmayer, entitled "Antistatika" includes a discussion of the application of materials for anti-static purposes. See for example:
(a) at page 306 Olsaurediathanolamid and the reference at the top of the right-hand column to Laurinsaurediathanolamid and Linolsaurealkylolamide;
(b) the bottom of page 420 and the reference to Aliphatische Sulfate and "hydrophobem";
(c) at page 421 right-hand column wherein the term "Hydrophilie durch Anhaufung hydrophiler Gruppen" is used; and
(d) at page 426 the disclosure of "Laurinsaure Sorbitmonolaurat" and "Glycerinmonostearat Athoxy Ette Glycerinmonofettsaureester".
U.S. Pat. No. 4,255,525 which corresponds to European Application No. 0009082 teaches the coating of small particles of expandable styrene polymer for moulding compositions by coating with betaines having the formula: EQU (R).sub.3 --N.sup.+ --CH.sub.2 --COO--
where R is the same or different and has 1 to 5 carbon atoms to prevent the build-up of anti-static charges of the surface. However, because the material disclosed in the Patent has very little hydrophobic material and considerable hydrophillic material, the material taught by the said Patent is too hydrophillic to be a suitable anti-staic agent. Furthermore, it is apparent that the process disclosed in the U.S. Pat. No. 4,255,525 cannot produce any suitable anti-static material because the drying of the material in the drying process will cause a loss of anti-static agent.
U.S. Pat. No. 4,302,549 is an interesting patent in that it teaches a process for the expansion of an expandable polymeric bead material by exposing the bead material to steam in an elevated temperature to affect the expansion of the material into a polymeric free-flowing particulate bead material while tumbling the bead material during expansion to prevent fusion. The process employs a lubricant for the bead material. In the process, the lubricant material (which is steam degradable) degrades on the exposure to steam so that its characteristics are substantially diminished in lubricant characteristics to provide an expanded polymeric foam bead material having a reduced surface lubricity. As is apparent, first of all the process requires water to break down the added material so that lubricity is reduced. As is apparent, the process of U.S. Pat. No. 4,302,549 cannot use powder as it needs moisture to get the effect claimed.
Consequently, the material employed would be too hydrophillic for use with comestibles, as for example, for a cup or container.
The difficulty with these approaches with respect to the manufacture of expanded polystyrene cups is that no expanded polystyrene cups suitable for use in association with comestibles have even been made by these methods.
Other patents dealing with the coating of beads are known although they do not relate to the use of an anti-static coating material.
U.S. Pat. No. 2,989,782 discloses a method for preparing moulded foam resin articles in which the surface shell resin differs from the chemical composition of the interior core of the resin particles but which does not disclose a finished anti-static article. Nor is any of the compatible organic compounds which are incorporated into the surface shell a good anti-static agent. The materials disclosed are too hydrophobic to constitute a suitable anti-static agent. Nor is there any disclosure of any process carried out to make an anti-static article.
U.S. Pat. No. 3,429,737 employs an amide of a C.sub.12-20 aliphatic hydroxy monocarboxylic acid as an anti-sticking coating for foamed polystyrene articles. No mention is made of anti-static properties of the material and there is none perceived. Additionally, the use of the coating material does not provide any anti-static properties. The compounds mentioned are essentially hydrophobic and thus would not confer anti-static properties.
Likewise, U.S. Pat. No. 3,480,570 discloses the use of a material (a surface active agent) that becomes incorporated into the particles of the polymer which is a suitable anti-static agent. Particularly, the materials proposed in this reference are too hydrophobic to be a suitable anti-static agent.
U.S. Pat. No. 3,560,414 discloses materials that coat the surface of the beads before impregnation with the butane which materials are too hydrophobic to be suitable. None of the materials disclosed therein is an anti-static agent.
U.S. Pat. No. 3,637,538 discusses the tumbling of foamable polystyrene particles with a specified coating agent, grinding them and then prefoaming the coated particles in a continuous prefoaming apparatus. However, the proposed coating agents cannot be anti-static since they are too hydrophobic. Where solutions are tumbled with foamable polystyrene particles to coat them and then dried, some if not a substantial part of the coating agent will evaporate and consequently, there will be very little of the coating agent left for uniformly coating the expanding particles.
U.S. Pat. No. 3,637,538 also teaches a coating material for coating styrene polymer but nowhere is there a suggestion of any anti-static material. Because the triester used has mostly ester molecules, it is most hydrophobic. The diester is substantially hydrophobic with the monoester least hydrophobic. The coating agent described in claim 1 is lipophilic, meaning "fat-attracting" so that minimal anti-static characteristics are provided.
U.S. Pat. No. 3,817,879 is to the same effect. Material that is too hydrophobic to be anti-static is disclosed.
U.S. Pat. No. 4,238,570 does not teach any anti-static materials which are used as a surface coating agent. This is clearly apparent from the disclosure because there are no hydroxyl groups in the molecules disclosed. The molecule is such as to be hydrophobic. Additionally, the process taught merely affects the shortening of the cooling time during moulding by using expandable particles of styrene polymer containing an ester of an aliphatic carboxylic acid with an aliphatic alcohol having no hydroxyl group in the molecule.
U.S. Pat. Nos. 3,908,069 and 4,312,957 do not disclose any materials possessing anti-static characteristics.
U.S. Pat. No. 3,908,069 merely provides coated expanded particles.
U.S. Pat. No. 4,312,957 provides for the incorporating of a synergistic mixture of surfactants into a polymer during the impregnation of the polymer with a blowing agent. There is no mention of anti-static characteristics. The material is provided solely for fast cooling.
French Application No. 2,404,022 does not disclose any anti-static properties in the lauric diethanol amide (Lankrostat JP) which is used only as an emulsifier for the incorporation of the blowing agent.
It is therefore an object of this invention to provide processes suitable for use in the manufacture of substantially static-free expanded, cellular or foamed thermoplastic cups and containers which overcome the aforementioned difficulties with the prior art and which processes employ anti-static agents suitable for use with comestibles.
It is a further object of the invention to provide anti-static foamed thermoplastic cups and containers suitable for use in association with comestibles.
Further and other objects of the invention will be realized by those skilled in the art from the following summary of the invention and detailed description of preferred embodiments thereof.